1. Field of the Invention
The present invention pertains to injection molded closures, and in particular, to such closures having means to prevent their jamming together when stacked one on top of the other.
2. Description of the Prior Art
Food products such as cottage cheese, sour cream dressing or the like dairy products are currently packaged in plastic containers having thermoformed plastic lids or closures therefor. These thermoformed lids characteristically have what is commonly termed a "plug fit", referring to a recessed plug-like central portion for insertion within, and to have tight frictional engagement with, the interior slanted surface of the container. The thermoformed closures require a secondary, post-forming operation after molding, in which an interior locking bead is formed on a deep skirt located at the upper end of the closure. The plug-like closures are distinguished from the closure of U.S. Pat. No. 3,933,264, for example, which has a circular central panel forming part of the uppermost surface of the closure. The central panel is joined to a collar-like container-engaging portion through an inclined wall, and by occasional triangular-shaped gussets which strengthen the connection between the upper panel and the outer container-engaging portion.
U.S. patent application Ser. No. 101,039, filed Sept. 25, 1987, discloses an injection molded closure which can serve as a direct replacement of thermoformed closures, and which is also suitable for use with existing packaging equipment designed for the mass, automatic filling and closing of thermoformed containers.
It is important that closures supplied for use with automated packaging equipment be arranged in a predictably oriented manner with respect to each other when placed in a stack. In particular, it is important that wedging between adjacent closures be avoided such that the existing packaging equipment for thermoformed lids can be used without sheet-down because of failure of the lids to be automatically removed and applied with sealing engagement with the filled container. As discussed in U.S. patent application Ser. No. 101,039 filed Sept. 25, 1987, the following U.S. patents are directed to lids which may be stacked one with another, but which are generally unsatisfactory for use with the container described in that application: 4,037,748; 4,046,282; 4,079,857; 4,103,803; 4,111,329; 4,165,020; 4,252,248; 4,418,833; and 4,682,706. The aforementioned U.S. patent application Ser. No. 101,039, filed Sept. 25, 1987, offered significant improvements in providing closures having protection against jamming when nested together in a stack, and further refinements are still being sought, especially with the increasing emphasis on automated packaging.